(Meth)acrylic acid and the commodity acrylate esters (methyl, ethyl, butyl, and 2-ethylhexyl) comprise one of the most versatile monomer series for controlling polymer performance characteristics. These monomers all have an alpha beta (α,β) unsaturated carboxyl structure and find extensive applications in surface coatings, adhesives and plastics. Furthermore, the sodium salt of polyacrylic acid is widely used as the superabsorbent polymer found in baby diapers. World production capacity for just crude acrylic acid is almost eight billion pounds per year.
The term “(meth)” indicates that the methyl substituted compound is included in the term. For example, the term (meth)acrylic acid represents acrylic acid and methacrylic acid, individually and collectively. While the process of the present invention can be employed in the production of acrylic acid and methacrylic acid, for the sake of simplicity the following description will refer to acrylic acid.
Currently, most, if not all, acrylic acid is produced commercially using a high temperature, two-stage air oxidation of propylene process. In the first stage propylene is oxidized with air to acrolein and then fed directly to the second stage in which the acrolein is further oxidized with air to acrylic acid. The catalyst used in each stage is a mixed metal oxide.
Acrylic acid is recovered from the product stream of the second stage reactor in a separation system. Various separation system designs exist. One such system comprises a quench tower coupled to an extractor which in turn is coupled to a series of distillation towers. The hot gases, e.g., gases at a temperature in excess of 230° C., exiting the second stage reactor are sent to the quench tower in which they are contacted with water. The hot gas condensibles, e.g., water, acetic acid, acrylic acid, etc., are separated from the hot gas non-condensibles, e.g., nitrogen, carbon oxides, etc. The off gases are sent to an incinerator and the cooled (e.g., a temperature of less than 230° C.) residue, liquid is sent to an extractor to remove the water. The extractor uses an organic solvent to extract the acrylic acid. The aqueous phase from the extractor is sent to a column in which residual solvent is azeotropically removed for recycle. The organic phase from the extractor is sent to the series of distillation towers from which crude acrylic acid is recovered.
As effective as these separation systems are, all are relatively expensive to build and operate, and all comprise a series of distillation columns that add to the complexity of their operation. Each tower requires the addition of fresh polymerization inhibitor at the top of the tower to prevent polymer fouling. The inhibitor is very expensive and adds to the production cost of acrylic acid.
In an effort to reduce costs and simplify operations, new separation systems have been proposed one of which is a coupled, two-tower system, i.e., a first or dehydration tower coupled to a second or finishing tower. The dehydration tower is equipped with a partial condenser which acts as a rectification system. The second or finishing tower is equipped with a reboiler and a total condenser, and it strips low-boiling and high-boiling impurities from acrylic acid product which is ultimately recovered as a side-draw.
Cooled (230° C. or less) gaseous reaction product from the second stage reactor is fed into a quench zone which can be located either within or without the dehydration tower (if located within the tower, then it is located in or near the base of the tower). Condensate from the finishing tower is used to quench the reaction gas into a concentrated acrylic acid solution. Vapor from the quench zone ascends into the dehydration tower. Acrylic acid is scrubbed out of this stream by the reflux descending from the top to minimize acrylic acid loss to the vent gas. Reaction water, acetic acid, light compounds and non-condensable gas are removed from the top of the tower some of which is recycled to the reactors and the remainder of which is purged to a thermal incinerator.
The bottom liquid stream from the dehydration tower is 70-100 wt % acrylic acid, 0-15 wt % water and 0-15 wt % acetic acid, and it is fed directly to the top or overhead of the finishing tower in which it is used as reflux. The finishing tower is operated under vacuum to prevent undesired polymerization of acrylic acid due to the relatively high (40° C. or greater) operating temperature of the tower. The overhead vapor is condensed and collected into a receiver, and the condensate is pumped and sprayed into the quench zone of the dehydration tower. Crude or technical grade (99+ wt %) acrylic acid is recovered as a vapor side-draw.
To obtain technical grade acrylic acid from the two-tower separation system described above, the water content of the tails stream from the dehydration tower should be between 1 and 15, preferably between 1 and 10 and more preferably between 3 and 5, wt %. If this water content is allowed to fall below 3 wt %, then the amount of acrylic acid loss in the overhead of the dehydrator is increased. If this water content is allowed to rise above 15 wt %, then the energy required to maintain reflux in the dehydrator is increased. Accordingly, tight control of this water content is important to the overall efficiency of the operation and to prevent undesirable polymerization and fouling of the equipment.